1. Field of the Invention
The present invention relates to a change-speed system for a utility vehicle, the system including a gear change-speed device operated for change speed in response to an operation of an actuator, a hydraulic clutch for selectively engaging/disengaging power transmission to the gear change-speed device, a current-controlled valve mechanism for feeding pressure oil to the hydraulic clutch, and a hydraulic clutch controlling portion for controlling oil pressure in the hydraulic clutch by controlling a value of electric current to the valve mechanism, wherein the hydraulic clutch controlling portion is operable to initiate a disengaging process of the hydraulic clutch based on initiation of the operation of the actuator and operable also to initiate an engaging process of the hydraulic clutch based on completion of the operation of the actuator.
2. Description of the Related Art
With a change-speed system for a utility vehicle of the above-noted type, the hydraulic clutch is temporarily switched over into the disengaged condition in operative association with an operation of the actuator for operating the gear change-speed device for change speed. Hence, this system can eliminate the trouble of manual clutch operation for temporarily breaking the power transmission in the course of a change-speed operation. Also, during a set period from the initiation of the engagement of the hydraulic clutch, the hydraulic clutch controlling portion maintains a predetermined high value for the electric current used for adjusting a valve opening adjustment to be supplied to the valve mechanism, so that with the opening degree being maintained at this large value, the oil pressure of the hydraulic clutch can be raised rapidly. Hence, in comparison with a case when the pressure of the hydraulic clutch is raised gradually and slowly from the initiation of the engaging operation of the hydraulic clutch, the time period required for engagement of the hydraulic clutch can be reduced advantageously. Furthermore, after lapse of the set period described above, the hydraulic clutch controlling portion temporarily drops the current value for the valve opening degree adjustment supplied to the valve mechanism for reducing the valve opening degree of the valve mechanism and then the pressure of the hydraulic clutch will be raised gradually, thereby to restrict occurrence of a shock at the time of clutch engagement.
Further, according to a change-speed system known from the Japanese Patent Application “Kokai” No.: Hei 7-208595, the system allows manual setting of the set period (so-called “initial period”) from the initiation of hydraulic clutch engagement for rapidly raising the pressure of the hydraulic clutch. With this construction, by manual setting of the set period, irregularity in the clutch engaging period which may differ from each other for individual hydraulic clutches for instance may be appropriately compensated for.
In the case of a utility vehicle, due to a change in the weight of the vehicle depending on its use condition such as the type of the implement mounted on the vehicle or secular change in the vehicle, there sometimes occurs a deviation of an initial clutch pressure generated in the hydraulic. clutch at the time of its engagement from a predetermined value. And, this deviation in the initial clutch pressure may provide a non-negligible difference between a subsequent actual clutch pressure and the preset clutch pressure immediate after the lapse of the set period. This results in a significant clutch engagement shock. In order to restrict this, it is necessary to adjust the clutch pressure immediately after the lapse of the set period, according to difference in the use condition and/or secular change. And, this adjustment of the clutch pressure too has to be coped with by the manual setting change of the set period.
However, the set period is the period for rapidly raising the pressure of the hydraulic clutch which is under the disengaged condition. With its change, the pressure of the hydraulic clutch immediately after the lapse of the set period too will be changed significantly. And, with such change, it is difficult to effect appropriately change of the set period, with the difference in the use condition and/or secular change, after the lapse of the set period. And, whether the set period is too long or too short, significant clutch engagement will occur. Further, even if the change of the set period can be appropriately effected for a change-speed operation to a certain speed stage, this can increase, rather than decrease, the clutch engagement shock, if the change-speed operation to a different speed is to be effected or if the change-speed operation to the same speed is to be effected, but this requires a different time. For these reasons, it would take considerable trouble and difficulty to make adjustment of change-speed shock according to difference in the use condition or secular change.